1. Field of the Invention
The present invention relates to a high frequency information detecting device for extracting high frequency information for auto focus from an imaging video signal, and an imaging device in which the high frequency information detecting device is installed.
2. Description of the Related Art
In recent years, there are things that can be described as brilliant in the transition from the analog technology to the digital technology in the camera industry. In particular, a digital still camera, which does not demand any film and developing process, has been more actively used, and a dominant number of portable telephones available in the market are now provided with a built-in camera. An auto focus function, in which contrast of an imaging video signal is used, is also installed as part of the digital signal processing, which is significantly improving in its speed and accuracy.
As recited in No. H03-96178 of the Japanese Patent Documents, for example, such a camera that executes a signal processing for the auto focus in which the magnitude of the contrast are utilized in high frequency information of the imaging video signal in terms of cost reduction is prevailing in the market. Further, in order to realize the auto focus function in a stable manner, an auto focus device for realizing the auto focus based on the center-weighted method by dividing a screen into a plurality of regions and separately weighting each of the regions was proposed.
FIG. 7A shows a constitution of a signal processing device including a conventional high frequency information detecting device 57 of the foregoing type. In FIG. 7A, 51 denotes a pre-processor for executing the black-level correction, gamma correction, gain correction and the like to an imaging video signal from an image sensor not shown, and 52 denotes a post-processor for generating a luminance signal and a color signal from the pre-processed imaging video signal. Describing the imaging video signal recited here, it is an output of the image sensor that is de-noised, amplified and A/D-converted. The image sensor comprises a filter of the RGB (red/green/blue) Bayer array.
In the high frequency information detecting device 57, 55 denotes a low pass filter for removing color carrier information resulting from the color filter in the image sensor from the pre-processed imaging video signal, and 56 denotes a high frequency data detector for extracting high frequency information for auto focus from the signal from which the color carrier information is removed. The high frequency data detector 56 extracts the high frequency information by each of the divided regions in the screen. 58 denotes a CPU for reading the high frequency information extracted by the high frequency data detector 56 and thereby executing feedback control of a lens mechanism part. The feedback control executed then is the auto focus control. FIG. 7B shows positions of the regions in the case where the number of the regions from which the high frequency information is extracted in the screen is assumed to be three.
In the high frequency information detecting device 57, a high-frequency color carrier component is eliminated by the low pass filter 55, and a main signal of the luminance signal is generated, and then, the high frequency information is extracted with respect to each of the regions in the screen by the high frequency data detector 56. The high frequency information is detected every frame. An average of dimensions summed in the three regions is used as a dimension of the high frequency information. Farther, the plurality of regions may not be three. As the divided regions are increased, the auto focus can be more stabilized.
In the conventional high frequency information detection, however, the average of the high frequency information extracted in the plurality of regions and summed is used for the auto focus. Therefore, it becomes difficult to place the focus on one point in the region at a high speed when a person is photographed in closeup or a still subject is macro-photographed. The averaging process is effective with respect to a photographed image having a large depth of field in order to assure an accuracy in the focus accurate, however, unsuitable for achieving a high accuracy and a high speed in the auto focus in a photographed image whose depth of field is small. A possible solution is to increase the number of the regions, which, however, invites increase of a circuit area.